1. Technical Field
The present invention relates to a refrigerant compressor, and more particularly, to a slant plate type compressor, such as a wobble plate type compressor, with a variable displacement mechanism, and suitable for use in an automotive air conditioning system.
2. Description of the Prior Art
Slant plate type piston compressors including a variable displacement or capacity adjusting mechanism for controlling the compression ratio of the compressor in response to demand are known in the art. For example, U.S. Pat. No. 3,861,829 to Roberts et al. discloses a wobble plate type compressor including a cam rotor driving device, and a wobble plate linked to a plurality of pistons. Rotation of the cam rotor driving device causes the wobble plate to nutate and thereby successively reciprocate the pistons in corresponding cylinders. The stroke length of the pistons and thus the capacity of the compressor may be easily changed by adjusting the slant angle of the wobble plate. The slant angle is changed in response to the pressure difference between the suction chamber and the crank chamber.
In a typical prior art compressor, the crank chamber and the suction chamber are linked in fluid communication by a path or passageway. A valve mechanism is disposed in the path and controls the link of the crank and suction chambers by opening and closing the path. The valve mechanism generally includes a bellows element having a needle valve thereon. The bellows is located in the suction chamber and operates in accordance with a change in the pressure in the suction chamber by expanding or contracting to move the needle valve into or out of a position where it opens or closes the path. That is, when the suction pressure is below a predetermined value, the bellows expands and the valve element closes the passageway, and when the suction pressure is above the predetermined value, the bellows contracts and the valve element opens the passageway.
When the passageway is open, the crank and suction chambers are linked, such that the crank and suction chamber pressures are generally equalized, and the slant angle of the wobble plate with respect to a plane perpendicular to the drive shaft increases. Therefore, the stroke length of the pistons increases towards the maximum value, and the capacity of the compressor increases as well. When the passageway is closed, the pressure within the crank chamber increases due to blow-by gas leaking past the pistons in the cylinders as the pistons reciprocate. The increase in pressure in the crank chamber with respect to the suction chamber pressure causes the slant angle of the wobble plate to be decreased, thereby reducing the stroke length of the pistons and decreasing the capacity of the compressor.
In this prior art, the suction pressure operating point of the valve mechanism at which it opens or closes the communication path is generally determined by the pressure of the gas contained within the bellows. Thus, the operating point of the bellows element is fixed at a predetermined value of the suction pressure. Therefore, the bellows element operates only due to a change of the suction pressure above or below the predetermined value, and is not responsive to various changes of the condition of the refrigeration circuit which includes the compressor, for example, changes in the thermal load of the evaporator of the refrigeration circuit.
One way of overcoming this drawback in the prior art is disclosed in U.S. Pat. No. 4,842,488 to Terauchi, which discloses a slant plate type compressor including a valve mechanism to control the communication between the crank chamber and the suction chamber through the communication path. The valve mechanism includes a first valve control device for controlling the communication between the crank and suction chambers. The first valve control device may be a bellows operating in response to the refrigerant pressure in the suction chamber. A second valve control device is coupled directly to the first valve control device, and controls the suction pressure operating point of the first valve control device in response to changes in external operating conditions, for example, the thermal load on the evaporator. The second valve control device may include an electrically activated solenoid. The current which is supplied to the solenoid, and thus the effect of the solenoid in changing the response point of the bellows, may be varied in accordance with the sensed external condition, for example, the thermal load of the evaporator. Therefore, the suction pressure response point of the bellows may be adjusted in accordance with the sensed external condition.
However, in the above discussed patent, the second valve control device is directly coupled to the first valve control device. Therefore, the effectiveness of the control of the operating point of the first valve control device which is provided by the second valve control device is reduced due to the inertial force generated by movement of the second valve control device, as well as the frictional force generated at the contact surfaces of the sliding portions of the second valve control device. Accordingly, the accuracy of the control provided by the second valve control device in adjusting the suction pressure response point of the bellows is decreased.